Only known chimp war reveals how societies splinter

PICTURE the scene&colon; a weak leader is struggling to hold onto power as ambitious upstarts plot to take over. As tensions rise, the community splits and the killing begins. The war will last for years.

No, this isn’t the storyline of an HBO fantasy drama, but real events involving chimps in Tanzania’s Gombe Stream National Park. A look at the social fragmentation that led to a four-year war in the 1970s now reveals similarities between the ways chimpanzee and human societies break down.

Jane Goodall has been studying the chimpanzees of Gombe for over 50 years. During the early 1970s the group appeared to split in two, and friendliness was replaced by fighting. So extreme and sustained was the aggression that Goodall dubbed it a war.

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Joseph Feldblum at Duke University in Durham, North Carolina, and colleagues have re-examined Goodall’s field notes from the chimp feeding station she established at Gombe to work out what led to the conflict.

In the past, researchers have estimated the strength of social ties based on the amount of time two chimps spent together at the station. But the notes are so detailed that Feldblum could get a better idea of each chimp’s social ties, for instance, by considering if the chimps arrived at the same time and from the same direction.

His team then plugged this data into software that can describe the chimps’ social network. They did this for several periods between 1968 and 1972, revealing when the nature of the network changed.

The results suggest that the Gombe community was united until 1971. Then the chimps suddenly split into two groups – one based in the north, one in the south – that spent less time socialising with each other. Feldblum presented the work at the American Association of Physical Anthropologists meeting in Calgary, Canada, last month.

It’s hard to say what caused the split, but a senior male called Leakey died at the end of 1970. “As soon as Leakey died they started splitting,” says Feldblum. “He seems to have been a bridge between the northern and southern chimps.”

After Leakey’s death a chimp called Humphrey became alpha male, but he was weak and faced pressure from two brothers from the south, Hugh and Charlie. The other chimps began to follow either Humphrey or the brothers. The battle began.

Over four years Humphrey’s group destroyed the brothers’ group, and the seven rebel males died or vanished. Groups of males would slip into rebel territory and savagely beat a single chimp.

“Groups of males would slip into rebel territory and savagely beat a single chimp”

It was possible to predict which group a chimp joined by looking at their preferred social contacts before the split, says Feldblum. This social fragmentation resembles human societies, he says, pointing to “an iconic study in sociology”, Zachary’s karate club, which showed how tensions among members of the club led it to split into two. Here, too, it was easy to predict how the group split.

That means chimp societies might help us understand how human-like societies evolve. More clues might come from New World spider monkeys, the only other primate that seems to behave similarly, says Anthony Di Fiore at the University of Texas at Austin. “There must be some ecological reason why they have converged on the same pattern of social organisation [as humans].”

Chimps have been known to attack humans, too. In Virunga National Park in the Democratic Republic of the Congo, chimps reportedly killed 10 people in 2012. Ongoing tensions between the chimps and humans may have triggered the killings.

All this suggests chimps and humans are cut from the same cloth, unlike peaceable bonobos, which are as closely related to us as chimps. That may be because bonobos have better access to easily digestible food, reducing the chance of conflicts over resources.

This article appeared in print under the headline “Secrets of the only known chimp war”